Postfix: Second sender address for a single mailbox

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http://serverfault.com – I have got two domains: a.com and b.com. Currently all mails to [email protected] are insert to the mailbox "a". I configure in the file /etc/postfix/vmailbox that all mails to [email protected] are forwarded to [email protected] Now I want to send mails from b.com, but postfix only accept smtp-requests from [email protected] How can I add a new SMTP account without creating a new "dummy" mailbox? Is there any possibility to create an alias for outbox-accounts? I'm using ISPConfig as configuration backend for postfix. (HowTos)